Hillary Clinton today raised the stakes in the confrontation with North Korea over the sinking of a South Korean warship, warning that Pyongyang would face international consequences for torpedoing the vessel.

Speaking to reporters during a brief visit to Tokyo, the US secretary of state refused to specify what action was being considered in response to the attack in March, which killed 46 sailors, but said the world would not revert to "business as usual" in dealing with North Korea.

"The evidence is overwhelming and condemning," she said during a joint press conference with the Japanese foreign minister, Katsuya Okada. "The torpedo that sunk the Cheonan … was fired by a North Korean submarine.

"We cannot allow this attack on South Korea to go unanswered by the international community. This will not be and cannot be business as usual. There must be an international, not just a regional, but an international response."

Clinton was in Japan for a few hours at the start of a five-day visit to Asia that will take in China and South Korea.

The White House has said it would prefer South Korea to take the initiative in deciding a response to the Cheonan sinking.

South Korea's president, Lee Myung-bak, today sounded a more cautious note in a statement released before he addressed an emergency meeting of the national security council. Seoul yesterday promised resolute measures against Pyongyang, but is wary of escalating the situation.

"Since this case is very serious and has grave importance, we cannot afford to have the slightest mistake and will be very prudent in all response measures we take," Lee said.

South Korean investigators, working with experts from several other countries, said there was overwhelming evidence that a torpedo fired from one of the North's submarines caused the blast that destroyed the Cheonan as it patrolled a disputed Yellow Sea border.

The North denies any involvement in the 26 March disaster. It repeated its warnings of war today and said it was prepared to tear up all agreements with its neighbour.

"From this time on, we will regard the situation as a phase of war and will be responding resolutely to all problems in North-South relations," the committee for the peaceful reunification of the fatherland said.

"If the South puppet group comes out with 'response' and 'retaliation', we will respond strongly with ruthless punishment including the total shutdown of North-South ties, abrogation of the North-South agreement on non-aggression and abolition of all North-South co-operation projects."

Separately, a North Korean naval spokesman, Colonel Pak In Ho, told the broadcaster APTN that the report's evidence was faked.

Pyongyang has said it will send its own team to investigate the incident, but an unnamed defence ministry source, quoted by the South's Yonhap news agency, said Seoul would not permit such a trip.

The issue, along with Iran's nuclear position, is expected to dominate Clinton's agenda.

Associated Press reported today that Seoul was pushing the Obama administration to put North Korea back on a terrorism blacklist.

But while the US government has condemned the March attack, a state department spokesman, PJ Crowley, noted: "Terrorism normally involves acts of violence against innocent civilians."

Although many expect Seoul to take the matter to the United Nations security council, experts say it would struggle to win new sanctions there. More likely would be a condemnation of the attack.

"Clearly this was a serious provocation by North Korea and there will definitely be consequences because of what North Korea has done," Crowley said.

But China, Pyongyang's key ally and a permanent member of the security council, has described the incident only as "unfortunate" and urged all parties to show restraint.

The 1950-53 war ended with an armistice, but the two Koreas never signed a peace treaty. Yonhap reported that the UN command's military armistice commission would investigate whether the North had violated the terms of the agreement to suspend hostilities.